Known industrial trucks normally have a vehicle frame and a lift mast with a load part. The load part and individual mast stages of the lift mast can be extended and retracted, while the lift mast can be tilted forward and aft and advanced and withdrawn on a reach carriage. Industrial trucks with a reach carriage are normally termed reach trucks. Moreover, known industrial trucks have a control unit for an operator to command a speed input which may be realized by a drive unit. For example, the lifting function, tilting function and thrusting function of the industrial truck can be realized by the drive. Generally, this is a hydraulic drive with one or more hydraulic cylinders. For example, to control the actual speed of the reach carriage, the control translates a specified speed into a manipulated variable corresponding to the characteristic of the hydraulic valve. The volumetric flow of the hydraulic fluid flowing into the hydraulic cylinder is controlled corresponding to this manipulated variable. The reach carriage at the foot of the lift frame is moved by the hydraulic cylinder, which causes a movement of the lift frame, and, hence the load. Unfortunately, the actual speed of the reach carriage can differ from the specified speed due to external influences such as production tolerances, fluctuating frictional forces, material wear or other static or dynamic forces.
Additionally, the thrust frame, and, hence, the load can experience undesirable vibration from the movement of the reach carriage. Such vibrations can be reduced by accelerating, or respectively, delaying the thrust mast gently in predefined operating positions. Such a method is, for example, known from WO 2008 006 928 A1. In this method the initial natural frequency of the thrust frame can be determined based on different operating parameters of the industrial truck such as the lift height and weight of the transported load.
Active mass vibration dampings are also known to measure variables proportional to the vibrations such as acceleration or a strain of the lift frame, and control the movement of the thrust mast based on these measured variables. Such active vibration damping is, for example, known from DE 10 2007 024 817 A1 wherein regulation occurs as a consequence of a movement characteristic of the thrust mast actuator saved in the control device for different operating situations of the industrial truck. Methods for active vibration damping are also known from EP 1 975 114 A1 and DE 10 2006 012 982 A1.
A need, therefore, exists for a control unit which ensures that the reach carriage, and, hence the load located on the lift frame, achieves a speed commanded by the operator.